Text in the following format is shorthand for the full Minecraft Item ID:
minecraft:item_name

Vanilla Behavior: (least in 1.18)
Using any non-empty bucket on any of the non-empty cauldron states, overrides the contents of the cauldron with the contents of the bucket.

Using any of the following bucket types:
water_bucket, lava_bucket, & powder_snow_bucket (too many fish bucket variants to list, also assuming fish buckets act like water bucket)
On any of the following cauldron states:
water_cauldron, lava_cauldron, & powder_snow_cauldron
Results in the type of bucket used becoming the corresponding cauldron state. In other words, contents of the cauldron is lost.

Feature Behavior:
When active only allow the following:
Empty bucket, bucket, to be used on water_cauldron, lava_cauldron, & powder_snow_cauldron.
and
Filled bucket variants, water_bucket, lava_bucket, powder_snow_bucket, & fish bucket variants, to be used on empty cauldrons, cauldron.

this also as you say it seems like a vanilla bug, did you report this on the Minecraft issue tracker? from what I see here you want a tweak to only allow opposing fill levels to interact, If vanilla Does not Fix this Bug(?) then it would be a great and maybe fun to implement tweak. also Sorry For the like 7 notifications masa

Thanks for the reply.
I haven't reported it on the Minecraft issue tracker yet. Knowing my luck Mojang will just say working as intended. xD
It's really annoying when I'm collecting lava and forget to switch to another stack of empty buckets, basically multiple lava sources have been lost do to the override. This also includes power snow.

How'd you like to proceed? The bug report first and then see where that goes?