Dear Daidai,
You sent your email when I just went on holidays.
You have been using the wrong value for argument 'n' in backtransf(). R package meta uses the harmonic mean of the sample sizes in the back-transformation of Freeman-Tukey double arcsine transformed proportions:
backtransf(test_meta$TE.common, sm = "PFT", n = 1 / mean(1 / test_meta$n))
backtransf(test_meta$TE.random, sm = "PFT", n = 1 / mean(1 / test_meta$n))
The use of the harmonic mean goes back to the publication by Miller (1978) - see help page of metaprop().
Best wishes,
Guido