That is not true. He does a great job when facing batters with RISP. He allow just 1 XBH all season in such (over 88PA). Last time he allow a few to scores, but that wasn't his fault.
There's a few reasons to bring in Tazawa with runners on, he almost certainly stop the running game. Nobody likes to get thrown out in key situations with someone who is quick to the plate as Tazawa.
Also his IR is 34 and 11 comes in. A few of those scored were by errors, and 2 last time because Farrell decided to put on the shift on what would been a inning ending DP. Tazawa also pitch with the bases loaded zero out 1 time as well.