It's not really surprising, though. Honest feedback can only benefit the seller and possibly disadvantage a buyer. After all, if the feedback is positive, the buyer is immediately in a weaker position vis-a-vis not only the seller but also the estate agent , through whom they may be looking at other properties, so that enthusiasm - or information about a willingness to pay £x - could follow the buyer to other viewings.
That is, of course, the best case. The worst case is what has just happened to us. We viewed a house and initially didn't want to make an offer, so our feedback may have been rather lukewarm (DH did it). Then DH went around with a friend who has a property project background and engineering in his family, and again, feedback was honest (critical, since the place has got some damp and cracks. Also, I could smell piss in the carpets, though no-one else, apparently, could!). We put in a low offer. Another family also bid. We counterbid twice, and were rejected, and although our last bid was close to the original, outrageous, asking price, the vendor has told us to stop bothering her! We believe our "honest feedback" offended her, and she is willing to get £15K less from the other family, who may also discover structural problems when they do their survey.
Given this, we're not going to give anything but bland feedback ever again, unless we don't want a house. We just don't trust vendors and agents not to turn it against us! OP, you may be a normal and reasonable person, but in a market like this, where there's so little available, we buyers are going to be careful with what few possibilities there are!