That is unusual. I’ve never messed with the Serpentis variety, but have killed many of all of the Sansha versions, and they can do some odd things. I used to hunt negotiators in a coercer, and it takes them around 8-10 seconds usually to lock you and start shooting. I currently use a harbinger, which generally they lock a little faster (maybe 5 seconds). Though i have had them apparently get “confused” and just let me pound on them to around 60% armor before they finally decide to shoot back.
With signature of a frigate being so small it should take them quite a while to actually target you. Though from the sounds of it i wonder if it was trying to start orbiting you, got trapped, then got stuck in some loop of trying to get into range so it it never tried to target you, I’m guessing it was a glitch.
I’ve never tried to speed tank them in a frigate, though I’m sure it could be done pretty easily, since orbiting them in the 500-1500M range in a dessie is enough to ignore most of their damage. Speaking of which there’s good money in the tags, especially the .2 and .1 versions, so if you’ve never hunted them you definitely should ;^).