You have experienced the Flushing effect caused by Niacin. Vit. B3. This is mentioned in the guidelines. This has nothing to do with an allergic reaction. https://microdosinginstitute.com/lions-mane-stacking/
Please take half a pill next time. If the flushing is still unpleasant, take a quarter of stop taking the Niacin.
This Flushing is normal and can do no harm and according to Paul Stamets this flush is the reason why psilocybin and Lion's mane pass through the blood/brain barrier, to the tips of the neurons, where neuroplasticity takes place. Nice story of course, but if you find it an unpleasant feeling, you can halve your intake of niacin. If you don't like it at all, you can also continue without the niacin.