Pixel 2 XL will not connect to low-signal wifi APs automatically, only manually

I have an issue that seems to have cropped up post-Pie. I know that WiFi issues have been prevalent with the 2XL, but until now I didn't seem to be suffering any.

Since Pie, my phone now has a problem holding weak WiFi channels, where it didn't previously, and will not connect to one automatically.

The scenario is that my home office has a couple of concrete walls between me and my router, this results in weak signal on our 5Ghz network but that doesn't matter to me, the performance is more than sufficient and I can generally still achieve around 100Mbps. My windows laptop and Asus Chromebook have no issue connecting, despite showing a weak signal, and have no issues with network performance. My Chromecast is on the same network, and never has issues with YouTube, Netflix etc. And until now, I never had issues with my 2XL, but now it practically refuses to work with the wifi network unless I force it.

The phone has been dropping the wifi and switching to LTE automatically, I only realized the problem was occuring when I saw the spike in my mobile data usage.

I know that previous versions of Android had options for things like Avoiding Weak Connections etc., but those seem to have disappeared. I've tried making some changes to the global settings for switches that I'm not even sure would still apply in 9.0, nothing seems to help.

If I select the network manually, the phone will connect fine and then work fine. If the connection drops, it will not reconnect.

If I'm in a part of the house with a much stronger signal, I have no issues at all.

Any ideas on what I can do to force Android to use a perfectly usable AP that happens to show weak signal strength?

I should add that I can also connect to a much stronger 2.4G network, but I can never hold it, and it blinks in and out of view in the available networks list. Not sure if that is Pie or the previously reported issues, I never used the 2.4G network previously so don't know if that behavior is new for me.

Any thoughts or suggestions appreciated.