MoinMoin!
There might be also another Problem within the clades of Hortiboletus: Those species (at least the three european species) seem to be really close. H. bubalinus might be the one with the statistically most seperated ITS, rubellus and engelii (formerly known as communis) seem to be very close and even vary in a range, that might suggest that it could be only one "species", but with quite a huge variaty in morphology as well as in molecular data. Within this clade, there could be two or three (sometimes!) distinguishable lines, but maybe these lines still are so close, that Hybrids (whatever that exactly means for mushrooms) could be possible - and confusing.
In quite a lot of older works, that are based predominantly on morphological features, H. rubellus und H. engelii (communis) are concidered to be only one variable species (e.g. Xerocomus - book of Fungi Europaei / Edizione Candusso).
If determinations of material are based on literatur like the mentioned, the names "rubellus", "engelii" and "communis" are more or less randomly mixed when checking for sequences. What might also be the problem within the unitee - database in this context.
So... If You're looking for matchings dna - data for a Hortiboletus like this one, You should blast the sequence (use GenBank!), sort out all "ectomycorrhizal samples" and ignore the "names" of the collection. What would be ideal: Search for type sequences (what should be possible with genbank) as well.
Lg; Pablo.