I've never had this happen with syrups (or 3787 for that matter). Doesn't mean it's impossible, I guess. Normally you'd point to yeast viability/quantity or aeration. I give.
How many of us ferment in a pretty much sealed environment, though? Although I my be imagining it to be more sealed than it is. But given that even an airlock can affect the performance of 3726, I have to wonder if that's at play here. I guess the OP could kinda "open up " his fermenter to see what happens.
Interestingly enough, if you read through the Westmalle section in BLAM, it states:
"Westmalle converted to closed fermentation 20 years ago." (2005 - 20 = 1985)
While not explicitly stated, the assumption is they were practitioners of open fermentation prior.
"They (fermentation tanks) are horizontal shaped, so the pressure column is very wide."
The assumption being they use long, wide horizontal closed tanks to minimize pressure and mimic the characteristics of open fermentation.
Don't know how that factors in ultimately.