That is indeed the root of the problem with all scanners that are analog devices not designed to properly demodulate complex digital signals. They can, under ideal conditions, get imperfect demodulation sufficient for trunk tracking and audio recovery for the non-critical scanner user. But, this fails with any disturbance in the received signal. So, directional antennas and finding a "sweet spot" location can mitigate, but never totally eliminate, the problem.
The Harris WCQPSK/H-DQPSK modulation actually seems to cause even more problems for scanners than Motorola LSM since the scanner can not properly demodulate either signal. I also found that OP25, at least in the early implementations I tried, had issues with the local Harris system as clearly seen in the scope patterns. When I tried my TRX-1 in the CNYICC area, it did better (but still marginal) than it ever did with the Harris Monroe/Ontario system.
The bottom line is that, to receive P25 simulcast systems reliably, the radio device must have proper demodulator circuitry or a SDR device with proper demodulation software. The Motorola and Harris digital signal modulation schemes that cause problems for scanners are intended to, and actually do, increase reliability and overcome simulcast issues in properly designed radios.