I'm not sure if this bug was recently introduced or if it's been this way, as I have only been recently leveling an Elec blaster. Tesla Cage in the electrical blast set has a duration of 8 seconds at level 1, and this does not scale at all with level. Most mez powers have a scaling duration that increases by 1% per level up, such that they are 49% stronger at level 50 (in this case, an 8 second hold at 1 would be 11.92 seconds at level 50). I checked with all versions of the power (sentinels, corruptors, defenders) and all of them show that the hold is only 8 seconds all the way to 50. The Electric control version of the power does not have this issue.