STEPS TO REPRODUCE:
1. suppose you have a project called "FOO"
2. create an issue within the "FOO" project ; this issue is auto-assigned a key/ID
3. let's agree this issue is assigned "FOO-100"
4. now Move FOO-100, but only change the issue-type (don't change it's project)
5. for example, move it from issue-type "Bug" to "Improvement"
6. now, create a second issue within the "FOO" project

RESULT:

• the second issue is auto-assigned a key/ID of "FOO-102"

EXPECTED RESULT:

• the second issue is auto-assigned a key/ID which is exactly [ the last issue key in the project + 1]
• in our case, we would expect the second issue to be assigned a key of "FOO-101"

WHY I EXPECT THIS:

• It can be very confusing for users to see a gap like this
• We often move issues within the same project, changing only the issue-type ; this results in many "gaps"

ADDITIONAL COMMENTS:

• Obviously, I do not know the JIRA data-base architecture at all (all I know is that mysql is used)
• However, if the issue-type is only a property of the issue (basically, a property of the object), why create a "null" entry in the db (the "gap" number) when an issue is moved? I assume this "null" entry is used as some kind of pointer...

• I apologize if this issue or a near-duplicate issue already exists within the JIRA public issue data-base. I queried the data-base a lot, but did not find a seemingly duplicate issue...