HOM Effect with Sector Portals. Can I get rid of stop it?

That kind of problem isn't inevitable with sector portals, but different engines seem to have different portal rendering capabilities.

For example, I've done scenes plenty more complicated than that in Eternity, but earlier today I was trying to help somebody on Discord solve a similar issue using LZDoom, and after going through all the old tweaks and hacks from EE's early days, I couldn't fix their rendering problem. Then out of curiosity, I converted the action/tagging setup to Eternity's format, and that engine displayed everything perfectly fine - no changes to architecture necessary.

If you're going to use Eternity, I could probably help set up the map correctly, but if it's for ZDoom or another engine, somebody else would probably know more. The main issues that caused problems like that back in EE's early days were 1) portal sectors needing different heights/textures than surrounding sectors 2) not enough map space in portalled area for camera to render full area 3) smaller portalled area too close to other interfering architecture. Not sure if any of that's applicable to you though, and I don't know how much help anybody could provide without being able to look at the map themselves, but no, there shouldn't be any reason for architecture like that to have those errors.

I suggest to upload the test map so someone can have stab at it, as HOMs can be caused with a handful of reasons.

My source port is old, and so is my computer. Someone already tested it for me and they everything is fine, it's just I'm using an old version of a popular source port. The latest version of GZDoom won't work on my computer because it doesn't have the requirements needed to run it. I need a new computer but can't afford one right now so I just have to make do with I have.