Hi, im just going to speak my mind so don't take it personally.
There are a number of red flags in your post.....
1) There is no reason to create a 70-day lease time.
2) There is no reason to build a 65534-host capable dhcp server on one network with a SRX240.
3) If either 1 or 2 is needed you have a design issue.
With that out of the way; Possible fixes/issue;
1) Make sure you are running the JTAC release version for the SRX240 - 12.1X46-D86 or 12.3X48-D85 as per Junos Versions
2) There was a problem report about DHCP leases being dropped during a reboot which might be the same for you, hence version ugrade suggestion. The issue applies to the new way of configuring DHCP but that does not mean old DHCP is not affected as well. DHCP PR1050723
3) If you need long leases, why not reserve them in DHCP?
# set system services dhcp static-binding 00:AB:CD:11:EF:GH fixed-address 10.10.1.100
4) If you don't want to do that and your configuration has a gazillion addresses, you can also use:
# set system services dhcp pool 10.0.0.0/16 maximum-lease-time infinite
5) View the address bindings (In CLI), reboot the SRX or restart DHCP and look at the leases again. Determine of the client are being assigned different addresses than before. If so, I would say you have the same issue as the problem report in 2)
> show dhcp server binding detail
> restart dhcp-service gracefully
Juniper/Microsoft/Citrix Engineer at IT Live Ltd, Auckland, New Zealand.
Sounds like a better solution for you would be to configure static dhcp bindings on the SRX. This will associate the same dhcp delivered address to the device and only to that device. This allows the devices to be dhcp but the address to be consistent across reboots. It works by associating the device mac address with the desired ip address in the configuration.