Yea that is exactly what I'm saying. I see the transition, sub, and get up window you mention after you block a transition attempt. I use this regularly to get up from transition spammers.
The issue lies with the striking on the ground. I have not noticed any "priority" allotted to the blocker of the transition when it comes to posturing up.
For instance lets use a example to clarify it better.
I'm on top in full mount
Guy on bottom just repeatedly keeps spamming L3 to get up.
I continue to block attempt after attempt with R2+RS Down
Eventually he burns all his stamina out while mine is still full.
Here is where the issues come in.
I can transition OUT of full mount after a block but that would be stupid.
If I try and stay in full mount the continue of spamming transitions means the ref will come in and stand the fight up, yet again stupid!!
Lastly, if I block a transition or get up attempt, then hold LS up to posture up. By the time my guy makes it to the sitting up position, he is able to spam yet another get up attempt. Now I'm in a position unlikely to block the attempt or land a strike. Then the get up attempt passes as a success and he gets up.
This is a constant issue with all ground positions, plus add couple more when you throw in sweeps.
A huge issue I see online against randoms is the ref issue. They will literally spam transition attempt after transition attempt. Back to back, back to back, back to back, back to back, back to.......ref comes in and stands fight. Rinse and repeat if you decide to take them down again. They want the fight standing and this is a exploit to get it. No matter how many blocks you do, or transitions you complete instead, it becomes fruitless to try and strike on the ground (only light strikes). So you can rule out posturing up, the transition speed is to fast.