This one is tricky. Q. tankata- is the cognate of N./S. tangad(a)- which is the normal example Tolkien used to justify the N./S. past tense suffix -ant, via analogy with half-strong past tense N./S. tangant and Q. tankante.

However, according to Q. descriptions, this verb would be expected to have a weak past **tankatáne. Thus I think tankata- actually represents conceptual vacillations on Tolkien’s parts from divergent ideas for the evolution of past tenses of Q and S, which he seems not to have resolved.

As evidence of this, see Tolkien’s aborted attempt to devise a new origin for the S past tense suffix on PE22/157:

Make a Q. past continuous. was eating. mātante. istante, oryante, ortante. (?More) like (?aorist) in syntax. and also helps to explain wide[?] use of -ant as past in S. [added underneath] better mātānē > mātane. istane (PE22/157).