We debugged and finally narrowed down the issue to FNCProgressBar. After replacing FNCProgressBar with TWebProgressBar, everything is ok without issue on version 2.5.1.
On an another test, we downgraded the TMS WEB Core to 2.5.0, we did not see this issue anymore for FNCProgressBar.
Unluckily, we can't replicate this on a demo, and we hope what we identified so far can help you to debug this a bit easier.
Unfortunately, we haven't been able to replicate the issue. Moving forward, we will be using WebProgressBar instead of the FNC one. Thank you for your response.
Additionally, are there any plans to make TAdvProgressBar compatible with web applications?