I simply doubt that both units are defective (DOA), as both are working on passive PoE injectors.
I just found the cAP-ac to be very tolerant, when it comes to PoE-specs....even with 12V/1A on passive injectors, it will start und run fine.
I never had problems with them in this regard, but I only have about three different brands of 802.3at/af injectors in use and hence experience with.
You could RMA the tenda or exchange it for another Switch or Injector from another brand or try to use this: https://mikrotik.com/product/rbgpoe_con_hp
This will convert a 802.3at/af injector-port into a passive injector. Unifi also has some of these, but with only half the wattage/Amps, I think.
The datasheet for cAP-ac is over exaggerating, when it comes to power draw....according to my Switch, it will go up to something short over 11W, but only on boot.
For example, this is cAP-ac, powered over 802.3at/af (via PoE 802.3at/af powered Switch DLink DGS-1100-05PD) *and* a PoE-Splitter on its ETH2 powering a 12V SIP Phone at the same time...both wireless interfaces on the cAP-ac are currently active with 5 clients and the Phone is connected (on-hook, though):
In short, I'd rather blame the tenda than the cAP-ac...based on my experience.