Steps to reproduce:
1) Create an item of any type and save it. For example "Zabbix agent".
2) Edit item and change type and save it again. For example change "Zabbix agent" to For example "Zabbix agent (active)".
3) Open item again and observe missing value from disabled field "Use custom multiplier".

It is possible to save the item, but the problem is:
if the item is made in template and template is linked to a host, then the templated item cannot be edited and will result with error: 'Value "" of "Custom multiplier" has incorrect decimal format.'