PMax tROAS change and Shopping campaign re-learning

How normal is it that when you lower tROAS, conversions start dropping? I'm trying to raise the ad spent of a PMax campaing because it isn't spending all the allocated daily budget and so I lowered the tROAS of 2800% I had set by around 10%. It is now spending more and more but the number of conversions is getting lower. Is it because Google is expanding its reach and targeting less qualified audiences, or something else?

Also, I'm in the process of separating this exact PMax campaign into a Mid/Low shopping catch all campaign, and I've been moving 40 of the least performing products a day. Currently they're products with no conversions and very few impressions for the past year, but that quantity is making the shopping campaign go into re-learning. Is there a maximum number of products I can add so that it doesn't go into re-learning, or as they are products with no conversion data, is that okay?

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