That's a bourdon tube gauge, a coiled tube "unwinds" some under pressure, moving the needle thru a linkage. If no leaks in the tube, it is incidental that changing it changed the engine running. The gauge is sprung from too much pressure on the bourdon tube permanently distorting it. Let's examine why. Either the gauge had an internal defect/weak spot, or the gauge saw a heavy pressure spike. The source of a spike is logically the fpr.

One, where exactly in the fuel system is the gauge?

Two, does the gauge remain stable at all running regimes?

To address some of the above, the only way you could suck air into the system is for the fuel pressure to drop below ambient, which will only happen if the fpr leaks down residual system pressure AND the fuel has vapor bubbles in it that recondense on cool down, causing a negative pressure, but I think the vapor pressure would increase as vac increases. In any case, the positive fuel pressure when running is a lot higher than the suction pressure possible in the line, so you'd see a leak under pressure before you'd get air sucked in.

I sort of tend to the obstruction in the line idea..