Mojave has never supported running on HFS+. It should be no surprise that doing so leads to unexpected behaviors, in particular with regard to software updates and installations.
Success !
I've been running Mojave 10.14.6 build 18G103 on a HFS+ formatted Samsung 970 EVO M.2 since it was released with out
any issues except this Security Update.
I will keep this newly APFS formatted SanDisk Z10 SSD as my " latest Mojave 14.6 backup " and also as a way to install future Mojave Security & any other important updates which are of course nice to have.
From tomorrow I'll CCC clone tonight's APFS installation to another HFS+ formatted SSD and see how stable it is over a month or so. Then, if there have been no issues, I'll clone that to the HFS+ 970 EVO.
chrfr Thanks so much for your assistance, much valued and very educational ?.
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OFF TOPIC
I'm extremely happy with Mojave 10.14.6 and won't even think about Catalina until it goes final.
( I even have my almost 16 year old Canon A3 format LBP-1610 laser printer running perfectly in Mojave with a Sandboxing / CUPs hack
this A3 printer is in 'as new' condition )