Tiara Tansducer issue

I guess we should start with where did you install the transducer? It should not be forward of the front of the motors. It should not be outboard of the motors. And it should not be aft of the struts. If you check those boxes I don’t know what to tell you.

If I recall, the B75 is a near flush transducer and subject to all kinds of turbulence and air bubbles. Hard to find a clean location for these. Down deep in the hull V, mid ship or further aft MAY help keep it in cleaner water. A protruding old style 'ducer usally works better at speed.

Back at my deck. Yes, The B75 is a near flush 'ducer.
It is also available in tilted models so even mounted at an angle of a V hull, the crystal is still pointing down.
This could be an issue also; are you sure it is pointing down?

It is the later B785 or this protruding style, that will offer the best bottom lock at high speed.
A fairing block is used to insure it points straight down, the crystal surface is in the water and turbulence /air is not going across it's face.
Not popular on trailered boats.

Was this DIY or dealer advised and approved install?

Well, yes, it is normal for loss of operation on a flush transducer, at speed, when subject to turbulence and air bubbles across its face..
Was this discussed during the sales/install interview?

Normal is not what you want to hear AFTER a hole is punched in your boats bottom.

How is this boat stored?
Trailer, lift, rack?
If clear of these, put in a real transducer or live with it.

B765 (or like) would be a good family of chirp-ready ducers to use.
The face is away from the hull surface and not going to get any air over it.