Zabbix agent uses old active metrics after rechanging item keys or making new keys same as old data. So, it starts to read the log files with old mtime and lastlogsize.

1. create item with key log[/tmp/foo.txt]
The agent reads the log files, sends the data to server and hold lastlogsize, e.g. 200.

2. change item with key log[/tmp/foo.txt, .*] for 1's items
The agent receives the item key and lastlogsize 200 and creates new active metrics. Then it has 2 active metrics following
i) log[/tmp/foo.txt] disabled lastlogsize=200
ii) log[/tmp/foo.txt,.*] enabled lastlogsize=200

3. some application outputs to the log file.
The agent reads the log files, sends the data to server and hold lastlogsize, e.g. 300. But agent holds other lastlogsize 200 for old item key log[/tmp/foo.txt]. So, current status is following.
i) log[/tmp/foo.txt] disabled lastlogsize=200
ii) log[/tmp/foo.txt,.*] enabled lastlogsize=300

4. create new item with item key log[/tmp/foo.txt]
After receiving the new item, the agent reads file form 201 bytes and send.

The problem is that agent judges the differences only with item keys. I suggest you to use itemid.