This is very likely the case. Node 15 is the new default.
This is one way to do it. I strongly suggest you stay with the Arduino IDE and not attempt to use platformio. The ‘obvious’ way would be to download the sketch into your laptop with the Arduino IDE on it, edit the default Node ID in the sketch and then compile and upload the changed sketch. You’ll also need to download and install some libraries in order to be able to compile the sketch. All the instructions are in ‘Docs’ here: Using the Arduino IDE — OpenEnergyMonitor 0.0.1 documentation
However, there should be an easier way. When you had your emonTx’s connected to your emonPi to update the sketch, you should have been able, under ‘Admin’ → ‘Serial Monitor’ to “talk” directly to the emonTx and configure the Node ID. which should now be stored (or capable of being stored) in EEPROM. The procedure is: Stop emonhub (the on-screen button) and start comms with the emonTx (another button). You might see something about “LCD”. Type ? and click ‘send’ to issue the command. The emonTx will give you a list of instructions for all the options available. You need n7 for the first emonTx, ( n8 for the second and so on ) and send that - it should respond and confirm, then importantly you need s to save the new NodeID to EEPROM. It should acknowledge this too.
(If it carries on printing lines like
OK 5 54 0 51 0 105 0 195 92 0 0 0 0 0 0 0 0 0 0 0 0 191 179 44 0 0 0 0 0 98 128 44 0 223 198 46 0 (-0)
OK 5 54 0 51 0 105 0 201 92 0 0 0 0 0 0 0 0 0 191 179 44 0 0 0 0 0 98 128 44 0 223 198 46 0 (-0)
to the window, you didn’t stop emonHub.)
You can restart emonHub, disconnect and reinstate your emonTx. Now, when you look at the emonhub log, you should see some data from Node 7, but as you have seen, it will be rejected because the data format is inconsistent.
What you probably have is your emonTx is sending data according to this:
[[15]]
nodename = emontx3cm15
[[[rx]]]
names = MSG, Vrms, P1, P2, P3, P4, E1, E2, E3, E4, T1, T2, T3, pulse
datacodes = L,h,h,h,h,h,l,l,l,l,h,h,h,L
scales = 1,0.01,1,1,1,1,1,1,1,1,0.01,0.01,0.01,1
units = n,V,W,W,W,W,Wh,Wh,Wh,Wh,C,C,C,p
Whitening = 1 [Whitening = 0 for all _rfm69n variants.]
(There are 40 bytes there - count the decimal numbers that follow “OK 7” but not the one in brackets, like in the example just above ‘If it carries on…’)
If you have 40 bytes, copy and paste the above into your emonhub.conf file (via Emonhub →Edit config in emonCMS), changing [[15]] to the Node ID you want: [[7]] for the first, and make sure there are no more entries with the same Node ID in the file, i.e. delete or comment out the old Node 7 entry with a # at the start of each line.
If your emonTx is not sending 40 bytes, tell me how many as I’ll need to think again 
)
MSG is the serial number of the message
Vrms the voltage,
P1 - P4 the powers in watts
E1 - E4 the corresponding accumulated energies in watt-hours (not kWh).
T1 - T3 the temperatures
pulse the accumulated count of pulses.