Hi,
The release strategy re-triggering depends on the release indicator used and it will re-trigger only if there is a value change or new strategy is determined. If you are simply flagging acknowledgement in a PO, it wont make any value change / new strategy and hence new release wont be triggered. Please check the note: 493900 - FAQ: Release Strategy Question number 3
Also, please check whether version management is activated for the PO document type. The release strategy will reset when version is set to complete and the new total net value is higher than the old net value. Refer the note: 1961482 - Reset release strategy when version number is set as Characteristic in Customizing
Regards,
AKPT