Yellow colour, before the reaction is stopped, is due to the formation of oxidized TMB, which isTMB(2+). The reaction is stopped by degrading HRP, but are there any further oxidations happening? what is the formula of this final product of bright yellow colour that we measure at 450 nm?
The following researchgate link will perhaps be useful to you.
https://www.researchgate.net/post/TMB_substrate_with_HRP_and_then_STOP_solution
It gets brighter? It shouldn't. That's the whole point of calling it a "stop" solution. Once you add it, the reaction is supposed to be complete and unchanging, giving you time to have a coffee break, and measure in peace.
Perhaps you're not mixing it efficiently. In that case, mixing by diffusion over time might make it appear as if the reaction proceeds over time. Mixing is not always practical, e.g when working with microplates. If that is the case, you can just add the solution and wait for mixing by diffusion to complete. Make your final measurement when it no longer changes.
If mixing is not the issue, then you must have some funny compounds in your sample that's causing unusual developments to occur. No idea what it might be. You'll need to use appropriate controls to try and pin it down (e.g. no TMB).
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