There have been some changes in the license management backend and some changes to our policy about stuff like that. There have been a few long discussions about it recently. This is probably the most informative: Virtualmin License: A problem was detected—your serial is licensed for only one server - #4 by Joe
Basically, because we don’t collect any data about what servers are doing (and we don’t want to collect that sort of data, we try really hard to respect privacy and never collect anything sensitive), we don’t have any reasonable way to enforce special cases for licenses like “I’m only using it for dev purposes”, so we’re not trying to do that anymore.
So, our general recommendation is “develop with GPL if you need something with no limits, time or otherwise” and if you need to test with Pro, you have up to 21 days of having it on a second server, which I think resets every month, so you can spin up a test container or VM every few weeks if you need to test Pro features, specifically. Since GPL and Pro share the same API and config files and you can backup and restore across them without much drama and the code is 90% the same, most people can develop/test on either without problem.