I've had the same problem with HeliBoard learning garbage. I just changed my settings though, and I think it should help:

1. Open HeliBoard settings
2. Open Text correction settings
3. Scroll all the way to the bottom, and turn off "Add words to personal dictionary"

If you scroll all the way to the top again, you can manually manage the personal dictionary, including adding words you do want, and deleting any junk that was added by mistake, before switching that setting off.