After updating from the previous version to 1.2.0 the server gives this error upon joining.

Registry remapping failed: Received ID map for minecraft:item contains IDs unknown to the receiver!
- srias_flowers:blue_poppy
- srias_flowers:hydrangae
- srias_flowers:orange_daisy
- srias_flowers:umbrella_leaf
- srias_flowers:crocus

official servers are on 1.1.1, so this would be expected

This is not an official server, this is my private server.
Perhaps the docker container has some issues in the upgrade process.. I'll double check that it actually did the upgrade.

did both client and server update to 1.2.0 and are the configs all the same? this is saying there are flowers on the server but not the client

Turns out that the docker container isn't smart enough to update the modpack with the new zip once the environment variable was changed. Will do a manual upgrade; I assume that'll fix the issue.

Just to confirm, the server was indeed not yet properly upgraded to 1.2.0. Having done that there were no further issues in joining the server.