Considering Mara's reaction to Bolaire being one of the Weapons of the Shapers' War, clearly Thjazi didn't tell the Cloak about Bolaire as more than someone who knows artifacts and has resources for them.
As for why Thjazi didn't bring Bolaire up, there's two ways I can see that going:
One: Thjazi did not know Bolaire's origins
In other words, Thjazi knew he was dealing with a sentient magic mask, but he did not know by whom or for what purpose Bolaire was created.
Bolaire was made disinclined to share anything with Thjazi that he didn't have to for fear it would be turned against him as more blackmail or something. And given his origin in the Shapers' War raises his value as a rare magical artifact, that seems like a smart secret to keep, even if it leaves Bolaire not knowing whether or not Thjazi actually knows.
Furthermore, the Panto seem to be a rather global secret. Hal had never even heard of what the halflings' weapons might actually be, despite being a history buff/nerd. And Bolaire has done his best to make any hints of the Panto even harder to find in Dol-Makjar in the past decade.
Two: Thjazi did not want to lose Bolaire as a resource
Bolaire was in a very useful position for dealing with all kinds of artifacts thanks to his position as a curator of the Archanade. Thjazi may not have been willing to lose that access to make the halfling bridge to the Piper's Down when/if he knew the Panto was multiple masks. Find one of the others and Thjazi doesn't have to lose a useful tool.
Or, if Thjazi was being nicer, he didn't want to cost Hal what Hal considered a friend that is family if he didn't have to. Hero calls Bolaire "uncle" too. If Thjazi doesn't have to damage his family to make a bridge, he won't. And maybe a pinch of wanting to leave the extra security on Hal and the kids that is Bolaire in place as well.
Both are interesting possibilities and I'll be interested to see if we get an answer on that front.