GIMP 2.10 a brush in the filter parameters

[Ofnuts]

I did the tests above with 2.10 of course. But yes, your problem is that whatever value is active (not None), whether is comes form a previous run or from your default value, is fed to some object that preset the value in the dialog and in 2.10 that fails.

Time to report the bug if you want it fixed for some future version (I expect 2.10 to have a new version every month for some time, because there are certainly other bugs...)