Yes, you are right to be concerned. I'm not saying for one moment that the work of TAs isn't sometimes invaluable. But they are not teachers. It's a distinct, separate, support role. TAs' work should complement that of teachers, not supplant it, but unfortunately the latter strategy isn't unheard of as a means of procuring cheap labour. And if it becomes commonplace, a decline in standards is probably inevitable. Sorry, but it is.
My DC's TA has helped her little phonics group come on in leaps and bounds with their reading. She's incredibly good. But small group teaching, whilst effective, is not the same as curriculum planning and differentiating for a whole class on a regular basis. Some are brilliant at supporting SEN children, or in a variety of different strengths. Even learning their own profession, under the supervision and guidance of already-qualified teachers, is a long-established practice and it works. No issues with any of that. But the supervision is key here.
Qualified teacher status exists for a reason. It's wrong enough that HLTAs are permitted or expected to teach whole classes on their own. But for TA without QTS to have sole jurisdiction over an entire Year 3 class is definitely not on, and as a parent I'd be asking questions. I'm also not sure it isn't breaking some official guideline, but am willing to be corrected on this.
The role of the TA is necessary to schools, but different from that of a teacher. YANBU.