@burnout426 Thank you for the reply and effort. Appreciated.
Forgot to mention that all my partitions are NTFS, if that has a bearing.
"download location" is set to 'Z:\Desktop', i.e. my desktop, and 'Ask where to save each file before downloading' is indeed checked because this is how I find it comfy.
My portable Opera is installed on 1 of the 4 partitions my SSD has. Those 4 partitions are - 1) Sys Reserved; 2) One OS; 3) Another OS to boot on another machine; 4) A work partition, which hosts portable apps and it stores Opera portable as well. It's important to note that this 4th partition is (unlike the first 3) a Logical Drive, whereas the prior are Primary Partitions. This, as I know, is how Windows partitions any partitions after the initial 3. The 4 partitions are contiguous, just in the order I mentioned them. With this explanation of my drive partitions, I contradict myself where I said in my last post that on my SSD I only have OS ... so, sorry for the confusion.
As to the portable edition of Opera I use - it is not the one you linked to but instead one from PortableApps.com.
I've not tried with another Opera portable to reproduce the issue. However, I do have another Opera profile on the side, but I almost never download/save anything from there. I did it as instructed here - https://forums.opera.com/topic/20040/how-to-set-up-multiple-profiles-in-opera - and I've not noticed the OP issue happening in the second profile, only in the first (main) one.
Yes, you are completely correct in your last paragraph.
If I'm the only one with the issue then it is on my end and has to do with the specifics of my machine/system, some of which @burnout426 correctly pointed out. In that case it's not Opera's fault. If anyone else gets this issue, however, I'd be happy to see them post here.
Thanks for the support so far, Opera team and community.