I am indeed seeing a performance impact when turned back on.
I'm seeing the Alerts logged recursively on Data Page load as the Data Page Load Activity is self referential, specifically, Step 4 of pyGetAllAttachments copies data into D_AttachmentList.pxResults rather than Primary.pxResults.
Changing that line, stops the infinite logging (until PEGA0044) although each time the harness is loaded it will log one (one for the list, one for the attachment count.)
This does however bring into question the accuracy of the developer help or of the functionality of 'reload once per interaction'.
Select the check box to cause the system to refresh the page exactly once per user interaction. This option is available only for data pages with Page Scope set to Thread or Requestor. When selected, the system ignores any values in the Reload if older than and Do not reload when fields at runtime .
A single load of the harness, according to the developer help, should only load this data page once.
My understanding is the fact that it references itself in the load activity shouldn't trigger a reload, let alone the list section and count section causing multiple loads as this a single user interaction.
I havent raised an SR as of yet, ideally the rectification is to leave it as OOTB.
If however (and that's a big if) you need it back to pre-upgrade behaviour, then the rectification would be:
Change Step 4 of pyGetAllAttachments to Page-Copy into Primary.pxResults rather than D_AttachmentList.pxResults stops the infinite logging (until PEGA0044,) although each time the harness is loaded it will log once for the list, and once for the attachment count.