I’m not sure what would cause that but maybe check that the “Time Remapping” isn’t changing the speed on you.

It should be Old: 100 and New: 100 by default (no speed change)
Failing that check if it happens on a brand new file.
If it doesn’t, something is up with the file settings. A cheap and nasty workaround (if that is the cause) would be to start a fresh file (relink the character) and then append the animation data.
Hope that helps.