Fail to reboot after recharge, ESP32/ESP8266 using a MPPT and 18650 battery

I try to build a solar energy operated project. I'm using an ESP32 to wake up every 45 minutes, measure bat's voltage using board's ADC, and send it to an excel sheet on Google drive.

The part of going to sleep, waking up, measuring and sending its value - is working flawless for some time in several other projects.

I want to describe the exact problem:

1. Installing the entire system: ESP32 with code to act as described.

2. using a 6W solar panel, 3.7v 18650 battery ( fully charged ) and a MPPT for charging the battery. All connected to the MPPT board connectors

3. System works fine for many days ( which means - system charges as expected more than a battery can hold ), until few days without sun, until battery got drained and stops waking up and doing its magic.

4. Now, after sun comes out, even for more that a day or two, system does not wakes up.

5. when checking battery level ( after more than a day in full sun ), battery level is low ( about 2.8- 3.2 v).

6. My assumptions are: a) board and code are working as expected ( at the beginning tests were with a wemos mini ). b) since system is working for days - no doubt that charging works - as seen in stored values in excel sheet. c) the fact that the battery gets drained - and perhaps I have to use a larger battery, does not change the physical behavior of what is the reason for it.

7. At the beginning - I was powering up the board via 3.3v directly from the MPPT connector. since it is un-regulated, I try to power it up via 5v pin, adding a buck boost converter. Still the problem persists - only after battery drainage

My guess and question for the forum here - perhaps during charging the battery (from 0v to what ever minimal value needed for board operation), board gets into some state that : a) not starting any valid boot operation b) consumes power c) not getting into sleep mode - since boot was not actually started d) even after getting fully charged battery - still board is stucked in that mode that is not sleep, or awake state, and stays somewhere in between.

Is it correct ? Is there a way to overcome a situation like this ( waking up after a battery is drained ) ? I'm not looking for an answer - " take a bigger battery :) "

Thank you ( and sorry for my English if it was not explained well )