There have been well documented issues of Avast Web Shield turning OFF automatically, generally on Samsung phones for a long time. There is no fix, just have to use settings to turn access OFF then ON again. See: Scan says Webshield off but Accessibility toggle for Avast is on
There is a specific forum section for Avast Mobile Security: Avast Mobile Security for Android - Avast Community
Note I am just another user not a Avast Team Member.