we have a problem that buyer comes and goes. it does not remain stable. It's like coming and going permanently. What can that be? the others are normal

Spigot-Version: CraftBukkit version git-Spigot-18e2b9b-2d9db47 (MC: 1.15.1) (Implementing API version 1.15.1-R0.1-SNAPSHOT)

Maybe something else is (occasionally) preventing this shopkeeper from spawning, or removing it. Were you able to resolve the issue?

after a new start it was stable but then I exchanged something that started to come and go again

What did you exchange? Did you consider some other plugin interfering with the spawning of shopkeepers? You could also turn on debug mode and see if something prevents the shopkeeper from spawning. But this will not detect if some other plugin remove its.

I was debugging now. Maybe you see the problem in it

https://pastebin.com/aTMxvQaK

22.12 16:05:58 [Server] Server thread/INFO [Shopkeepers] Shopkeeper (world,258,71,195) missing, triggering respawn now
22.12 16:05:58 [Server] Server thread/INFO [Shopkeepers] Chunk was silently unloaded before! (dead: true, valid: false, chunk loaded: true)
22.12 16:05:58 [Server] Server thread/INFO [Shopkeepers] Shopkeeper (world,258,71,198) missing, triggering respawn now
22.12 16:05:58 [Server] Server thread/INFO [Shopkeepers] Chunk was silently unloaded before! (dead: true, valid: false, chunk loaded: true)
22.12 16:05:58 [Server] Server thread/INFO [Shopkeepers] Shopkeeper (world,258,71,186) missing, triggering respawn now
22.12 16:05:58 [Server] Server thread/INFO [Shopkeepers] Chunk was silently unloaded before! (dead: true, valid: false, chunk loaded: true)
22.12 16:05:58 [Disconnect] User Janis6903 has disconnected, reason: Disconnected
22.12 16:05:58 [Server] Server thread/INFO [Thirst] Saving player Janis6903
22.12 16:05:58 [Server] Server thread/INFO Janis6903 hat Lululand verlassen.
22.12 16:06:18 [Server] Server thread/INFO [Shopkeepers] Shopkeeper (world,258,71,186) missing, triggering respawn now
22.12 16:06:18 [Server] Server thread/INFO [Shopkeepers] Chunk was silently unloaded before! (dead: true, valid: false, chunk loaded: true)
22.12 16:06:18 [Server] Server thread/INFO [Shopkeepers] Shopkeeper (world,258,71,198) missing, triggering respawn now
22.12 16:06:18 [Server] Server thread/INFO [Shopkeepers] Chunk was silently unloaded before! (dead: true, valid: false, chunk loaded: true)
22.12 16:06:18 [Server] Server thread/INFO [Shopkeepers] Shopkeeper (world,258,71,195) missing, triggering respawn now
22.12 16:06:18 [Server] Server thread/INFO [Shopkeepers] Chunk was silently unloaded before! (dead: true, valid: false, chunk loaded: true)
22.12 16:06:20 [Server] Server thread/INFO Ollysan issued server command: /home lager
22.12 16:06:20 [Server] Server thread/WARN Ollysan moved too quickly! -3381.7666386603482,-12.899999999999999,1559.341845727299
22.12 16:06:28 [Server] Server thread/INFO [Shopkeepers] Shopkeeper (world,258,71,198) missing, triggering respawn now
22.12 16:06:28 [Server] Server thread/INFO [Shopkeepers] Chunk was silently unloaded before! (dead: true, valid: false, chunk loaded: true)
22.12 16:06:28 [Server] Server thread/INFO [Shopkeepers] Shopkeeper (world,258,71,195) missing, triggering respawn now
22.12 16:06:28 [Server] Server thread/INFO [Shopkeepers] Chunk was silently unloaded before! (dead: true, valid: false, chunk loaded: true)
22.12 16:06:28 [Server] Server thread/INFO [Shopkeepers] Shopkeeper (world,258,71,186) missing, triggering respawn now
22.12 16:06:28 [Server] Server thread/INFO [Shopkeepers] Chunk was silently unloaded before! (dead: true, valid: false, chunk loaded: true)

Looks like some other plugin might be removing those shopkeepers there.. (indicated by dead: true, but chunk loaded: true).

Do you run any plugins which might remove entities (for example performance-enhancement plugins, mob-spawn-prevention plugins, etc.)?

i think the problem is solved. I use the Mobfarm Manager. I have now set villager in it to -1 now it seems to remain. Funny thing is that only the buyer was affected. the normal sellers all worked

Maybe it is coincidence that it only affected the buyer shopkeepers, due to the order in which it counts the nearby villagers. Maybe you could ask the author of this plugin to ignore plugin-spawned mobs (or add a setting to ignore them). Shopkeepers for example adds the metadata tag 'shopkeeper' to every spawned mob. So if this plugin offered a config setting of the metadata tags to ignore, you could configure it to ignore the shopkeepers.