I am an electromagnetic compatibility engineer. To determine if the problem is weak signal or multipath interference takes a device to measure field strength at the specific frequency like spectrum analyzer or a receiver with a field strength meter, and a highly directional antenna.
That said, it is highly unlikely that the problem is multipath. The interfering multipath signal would have to have a signal less that 3dB less than the primary signal. It is not that it can’t happen, but it is unlikely.
The new metal structures could block the signal to a degree and may have interrupted the line of sight between the transmitting antenna and the receiving antenna.
TV signals, even VHF (lower channels 2 - 13), are very much line of sight. Antenna height is critical. Lack of height can be mitigated by a high gain antenna, rotor, and a low noise preamp, and high quality, low loss RF cable.
Yes, I remember that 3 dB capture ratio figure as being typical of cheap FM radio receivers. But, I probably do have some blockage and who knows what sorts of weird nodes could be set up by 4 large metallic structures...
I also remember how much reception could vary from room to room in tall apartment buildings in clusters (of buildings) even when signal seemed strong. (They were dorms some of my classmates lived in.) I actually had a high end FM tuner, Onkyo, IIRC, which had both a genuine signal strength meter and a dedicated output with which one could monitor multipath.
Anyway, even with some blockage I’m having trouble accepting the idea of a weak RF field from a 155 KW TV station (806 ft. antenna) under 7 miles away. The more distant stations... yeah...