First witnessed in new tileset and later tested in simulacrum as both host and client and I've found out that the attack speed buff works as intended when: You're the host (whether you have tennokai equipped or not), or when you're a client without having tennokai equipped. I tested it only with Mentor's legacy because I don't have them yet, so I couldn't test if this applied to those as well.
When you have it equipped the buff either doesn't register or it runs out prematurely. I tested with only a single instance of the buff (because you can stack it individually and I wanted to get some concrete buff durations) at 12x, and my attack speed reverted to default at random intervals while the buff still had duration left; sometimes it would revert when I had 5 seconds left and others 20.
I don't know exactly why the buff reverts, maybe the actual buff timer is shorter than the HUD suggests or, more likely, tennokai overwrote or cancelled on-heavy-attack effects? It's reproducible for me so I can only expect it to be for you as well.
Edit: For clarification the special effect is bugged for both tennokai heavy attacks and normal heavy attacks