My seven level 13s took on a 19-solo beholder last week (2nd enc of the day, after dealing with some ogre and drakkoth lackeys) and we had one actual death and a lot of scary times. They went in prepared, which helped, but the party got split as the beholder telekinesed 3 of them down a 75' well so it could focus on the other 4.
In my other campaign, six level 4s fough kruthiks and came as close to a TPK as I've seen, even though it was just two L+0 and one L+1 encounter. Kruthiik hive lord is seriously bad news, with an at-willl weaken/ save ends blast AND an allies-double-damage aura.
I think somehow you are being too kind to your players.