Yes, modifications in GTM can have an impact on the insights derived in Google Ads. Altering the GTM configuration can influence how data is gathered and transmitted to Google Ads, potentially affecting the algorithm's capacity to optimize and make informed decisions based on historical data.
In the specific situation you outlined, changing the pageview trigger in GTM from landing page A (/thank-you) to landing page B (/thank-you12) has the potential to reset the learnings in Google Ads. The system may treat the new landing page B as a distinct entity, and the historical data linked to the previous landing page A might not be retained. Consequently, this could disrupt the algorithm's ability to comprehensively understand user behavior and conversion patterns.
The observed significant decrease in recorded conversions, following the switch from thank-you page A to B for your conversions, indicates a negative impact on the performance of your Google Ads account due to the change.