show tag values unexpected behavior

__scenario__

__query:__
`show tag value with key="mykey"`

__Actual and expected behavior:__
```
name: data
key value
--- -----
mykey aaa
mykey bbb
mykey ccc
.....
```

__query:__
`show tag value with key="mykey" where mykey='aaa'`

__Actual behavior__
empty

__Expected behavior__
```
name: data
key value
--- -----
mykey aaa
```

__query:__
`show tag value with key="mykey" where mykey=~/^a.*/`

__Actual behavior__
empty

__Expected behavior__
```
name: data
key value
--- -----
mykey aaa
```

__query:__
`select * from data where mykey='aaa'`

__Actual and expected behavior:__
NOT empty

__additional notes:__

1. the problem seems to affect the series which have "holes" in the time span i.e. maybe not all the tsm files contain all the series
2. the problem vanishes after a restart

__Environment and System Info__

system: debian stretch Linux 4.9.0-6-amd64 x86_64
InfluxDB: 1.5.2
index-version = "inmem"
db with a 30d retention policy

Start at the SHOW TAG VALUES query path and compare its filtering with SELECT handling, focusing on series with holes across TSM files. Reproduce the literal and regex queries in the reported InfluxDB 1.5.2, 30-day retention setup, then verify both return aaa consistently without requiring a restart.

Written by the indexing model from the issue text.