The OS is Windows 8.1 Pro 64 bit with Update 1. I installed 2018 over-top via avast update. Defender is disabled (always has been). Windows Update never showed Def updates before v2018 and they won’t install anyway since it is disabled. In sure that 2018 is the culprit since it is the only changed I have made to my system recently. The difference is that with 2016, a white box popped up to say that Defender is turned off and with 2018, the Defender UI pops up.
I did run services.msc and saw 2 entries for Windows Defender. With 2016 - the start-up type is Manual. With 2018 - I believe the start-up type on one of the entries is Automatic. This may have something to do with it. I can’t change it since it is disabled.
I have restored a drive image with v2016. I hope that you can confirm this problem and release a fix. Thanks.