Inconsistent sleeping with SDK 2.1.0 uint64_t sleep times

I'm trying the extended deepSleep times available with the latest SDK 2.1.0 from https://github.com/esp8266/Arduino/pull/3278

Some sleep time values work fine but some don't seem to work and the ESP wakes up almost immediately. Bit strange. Here's a sketch that shows the problem:
```
extern "C" {
#include "user_interface.h"
}

void setup() {
Serial.begin(115200); Serial.println();
Serial.print("rtc proc: "); Serial.println(system_rtc_clock_cali_proc());

// uint64_t sleepTime = 60000000; // WORKS: 60 seconds
// uint64_t sleepTime = 7200000000; // WORKS: 2 hours
// uint64_t sleepTime = 9000000000; // WORKS: 2.5 hours
uint64_t sleepTime = 21600000000; // FAILS: 6 hours
// uint64_t sleepTime = 74049000000; // FAILS: bit over 20 hours
// uint64_t sleepTime = 86400000000; // WORKS: 24 hours

Serial.println("deepSleeping ...");
system_deep_sleep(sleepTime);
delay(100);
}

void loop() {
// shouldn't get here
Serial.println("loop");
delay(1000);
}
```