Bid-ask spreads for UCITS ETFs holding US equities

Conventional wisdom dictates that the bid-ask spread for a UCITS ETF traded on the LSE, that holds primarily US equities, should be lower when both the LSE and NYSE are open, and higher when only the LSE is open. I, however, found this was not the case for the AVGS UCITS ETF, around 70% of whose holdings are US equities. I recently switched from holding AVUV and AVDV in the NYSE, to AVGS in the LSE, to avoid estate tax implications in the US, and to take advantage of the accumulating nature of A