Right up to polling day the opinion polls reported that the Gorton and Denton By-Election was a narrow three way race between Reform, the Greens and Labour. In fact the Greens had a 4,000 majority over Reform.
Even Omnisis which had the Greens at 33% was way out.
The likely issue (Times 28th February) is that while the pollsters found it relatively easy to seek the views of white working class voters they struggled to judge the views of Muslims, other ethnic minorities and those switching from Labour to Green. They were fairly accurate on support for Reform and Labour but not the Greens.
Green voters it seems are part of a world of which pollsters have little conception
Kmflett's Blog 
On the other hand, the bookies got it bang on. I looked at Betfair, other sitez were the same or similar. All had the Greens as massive favourites with Reform a distant second and if anyone wanted to throw their money waway the odds against Labour were very good indeed.
Follow the money.
I particularly like that last paragraph.