This question has been discussed at length over on the iPhone 7 device support pages. Since the 7, 7 plus, 8 and 8 plus phones all take the same button, it's definitely applicable.
SOLVED: home button replacement assistive touch won't go away - iPhone 7 - iFixit
While several workarounds were discussed, there were two solutions that came up in this question. The first was a specific brand of home button - JCID - that does not trigger the assistive touch. The second was that it appears to be related to an operating system update; it's unknown when it went into effect, but according to one user upgrading from 15.7.5 to 15.7.7 eliminated the problem for them.
Good luck; this still very much a live issue so let us know what you find and if one of those solutions works for you.