@tut I noticed that char-limit parameter is missing from config.toml. Now this limit should be retrieved from the instance, but for mine it doesn't work.
It's nothing custom — just base Pleroma 2.5.0 instance. Both PleromaFE and Tusky detect the limit fine, tut fails on this condition in GetCharLimit() api/instance.go: "if ac.InstanceOld == nil || ac.InstanceOld.Configuration == nil…" and just goes with 500.
Looks like a problem with underlying module, I didn't investigate further.