SUMMARY
When saving a new animated brush tip, the names of the Selection Mode are wrong. Each one actually saves to another Selection Mode (constant saves to random, pressure to increment, etc.)

STEPS TO REPRODUCE
1. In a new document (64*64 for example), create ten concentric full circles on different layers, each bigger than the previous one, and remove the default layers (the empty one and the white one)
2. Open the Brush Editor (f5); select a brush using the pixel engine, and uncheck all of its settings that use the Pen Settings (it is especially important that "Size" be unchecked, for legibility of the bug)
3. In "Brush Tip" > Predefined, click "Stamp"
4. Name your stamp "test_const", set Style to "Animated" and Selection Mode to "Constant", then click ok.
5. Redo 2. and 3.. Name your stamp "test_rand", set Style to "Animated", Selection Mode to "Random", click ok.
6. Redo 2. and 3.. Name your stamp "test_press", set Style to "Animated", Selection Mode to "Pressure", click ok.
7. Redo 2.. In "Brush Tip" > Predefined, select each of your stamps in turn while paying attention to the brush preview in the upper left.
8. (if you want) redo 7. but instead of looking at the brush preview, between each change of stamp, try it out on the canvas.

OBSERVED RESULT
with "test_const", the brush preview looks like random ink drops, with "test_press" it shows a pattern like several slugs (small, getting bigger, then suddenly small again, growing again, small, progressively bigger...).

EXPECTED RESULT
with "test_const" it should always have the same size, and with "test_press" it should be small on the left and progressively bigger going to the right.

SOFTWARE/OS VERSIONS
Fedora 27