Did you check the setting at App Settings > Experimental features > Enable app embedding user experience? I used this on an app (not a customized form) and it did align the app to the left which when I embedded it using an iFrame, was what I wanted. Maybe this got turned on for your form somehow.
Do you create a custom form for your SP list using PowerApps or create an app based on the SP list?
Do you embed an app within a SP Site Page?
Could you please share a screenshot about your issue?
If you create a custom form for your SP list using PowerApps, the PowerApps form would be right aligned in default, regardless of, the "Enable app embedding user experience" option is turn on or turn off within Advanced settings of App settings.
If you create an app based on the SP list or embed an app within a SP Site Page, please check if you have enabled/turned on the "Enable app embedding user experience" option within Advanced settings of App settings, if yes, please disable/turn off this option, then re-load your app to check if the issue is solved.
Yes, it is the 1st option: custom SP list form using PowerApps.
I force opening the form in full page and it used to be centered (default behaviour without any customization).
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