Ran into a similar issue today, and based on my research, I believe the issue is that Alpaca is using the Exchange provided timestamp for their time windows, while Polygon is using the SIP timestamps.
While Polygon provides both the SIP and Exchange timestamp, Alpaca unfortunately only seems to provide/use the Exchange timestamp. This causes some quite significant discrepancies in how bars are aggregated thus leading to incorrect prices and volume. I will say that the full volume of trades do seem to exist, but just not in the correct bars. i.e. on 5/4/2022 the 8AM EST bar for TQQQ is showing around 200k volume on Polygon/TradingView, but only ~20k (10%) on Alpaca. However, if I look at the volume of trades spanning 7AM EST to 8AM EST, the total volumes are more or less the same. Unfortunately Alpaca isn’t just off by 1 or 2 bars, sometimes the trades are an hour off compared to Polygon.
See my forum post for details: Invalid bars around 8am EST shuffle for TQQQ - #3 by CapitalMastery
I currently have a ticket open with support, but I’m not feel very hopeful. I get the feeling that the support person doesn’t understand my issue, and for whatever reason isn’t able/willing to pull in someone from the data team to provide support.
Alpaca could be the perfect offering if they get these data issues fixed. I’m currently paying $200/mo for Polygon, and I’m really hoping I can switch to Alpaca sooner than later.